Clinically outdated human platelets were extracted with acid-ethanol and precipitated with ether. The extract induced NRK-fibroblasts to undergo anchorageindependent growth. Addition of EGF to the incubation mixture increased the potency of the extract by 1000-fold; EGF alone had little biological activity. Gel filtration of the extract on Bio-Gel P-60 revealed a single peak of biological activity having an apparent molecular weight of 20,000. The elution position of this peptide was distinct from that of radioiodinated PDGF. These findings demonstrate the presence of TGF-beta in platelets.